This invention relates to adjustable high voltage power supplies and more particularly dc-dc resonant converters suitable for powering X-ray tubes.
High voltage adjustable power supplies with a power output from a few hundred watts for applications such as dental X-rays to the 100 kilowatt range for diagnostic modalities such as computer tomography and X-ray fluorography, are used to energize X-ray tubes. A power supply with low weight, low output ripple to reduce filtering requirements and fast speed of response to accurately set exposures is desirable. Conventional power supplies of the single and three phase versions for energizing X-ray tubes use 60 Hertz adjustable autotransformers to supply adjustable voltages to a step-up 60 Hertz transformer. The 60 Hertz transformers are bulky and the prior art circuits have high output ripple and slow response.
U.S. Pat. No. 4,295,049, of Ebersberger et al, discloses a resonant circuit using silicon controlled rectifiers (SCRs). The resonant circuit is not configured to take advantage of the capacitance built into the transformer. Also, voltage control is achieved by a phase controlled rectifier front end since only limited voltage control can be achieved by lowering the SCRs frequency. The frequency can only be lowered from resonance and not raised above resonance since a leading load is needed to maintain SCR commutation. Operation at frequencies below resonance results in increased output ripple and requires the transformer to be designed for the lowest operating frequency to be encountered. The lower the frequency the larger the reactive components required.
It is an object of the present invention to provide a high dc output voltage from a converter having a low weight compared to designs using 60 Hertz transformers.
It is a further object of the present invention to provide a high dc output voltage from a converter with low output ripple and fast speed of response.
It is another object of the present invention to minimize the number of power components required in a high dc output voltage converter of a given rating and thereby increase reliability and achieve lower cost.